Generally, the time sharpness of a photographic light-sensitive material is reduced as one increases the thickness of any emulsion layer(s) therein because of light scattering by the silver halide emulsion grains. Particularly, in a multilayer color photographic light-sensitive material having a red-sensitive emulsion layer, a green-sensitive emulsion layer, and a blue-sensitive layer, light scattering is increased in a cumulative fashion because of the multilayer structure thereof, and a reduction in sharpness becomes particularly high in the lower emulsion layer(s).
In U.S. Pat. No. 3,402,046, a method for improving sharpness where coarse grains having a particle size of 0.7 micron or more which cause less light scattering are used in the blue-sensitive emulsion layer of the uppermost emulsion layer in a multilayer color photographic light-sensitive material is described.
Further, in U.S. Pat. No. 3,658,536, a method for improving sharpness where one of two blue-sensitive emulsion layers is positioned below a green-sensitive emulsion layer or a red-sensitive emulsion layer is described.
However, these methods have the disadvantages that the graininess of the blue-sensitive emulsion layer is deteriorated because of using coarse grains having a larger particle size than required as the blue-sensitive emulsion grains.
In U.S. Pat. No. 4,439,520, a color photographic light-sensitive material having improved sharpness, sensitivity and graininess, where tabular silver halide grains having a thickness of less than 0.3 micron, a diameter of at least 0.6 micron and a diameter/thickness ratio of 8:1 or more are employed in at least one of the green-sensitive emulsion layer and the red-sensitive emulsion layer is described.
However, in the case of employing such tabular silver halide grains, although sharpness in the high frequency area is improved, sharpness at a low frequency area conversely tends to deteriorate. The term "low frequency area" as used herein means an area having a frequency of less than 10 cycles/mm and the term "high frequency area" as used herein means an area having a frequency of not less than 10 cycles/mm.
With tabular silver halide grains, high reflection on the surface of grains is pronounced while they have less scattering of light passing therethrough. Due to their strong light reflection, multiple reflection occurs between a tabular grain silver halide emulsion layer and the surface of a film or between a non-tabular silver halide emulsion layer and a tabular grain silver halide emulsion layer when the non-tabular silver halide emulsion layer is present between the tabular grain silver halide emulsion layer and the surface of a film. Further, in the tabular grain silver halide emulsion layer itself, multiple reflection occurs between tabular silver halide grains since the tabular silver halide grains are generally distributed substantially in two or more layers.
Such light reflections have a tendency to deteriorate sharpness at the low frequency area rather than at the high frequency area.
The ratio of reflected light to transmission light not absorbed varies depending on the thickness of the tabular silver halide grains. The variation of reflectance against wavelength depending on the thickness of a silver bromide sheet in the area including is described in Research Disclosure, No. 25330 (May 1985).
It is difficult to reduce the reflection from the surface of tabular silver halide grains having a thickness of 0.07 .mu.m to 0.16 .mu.m over all wavelength regions of visible light.
For instance, when tabular silver halide grains which reflect blue light, but do not substantially reflect minus blue light, are employed for the purpose of improving the sharpness of lower layers in a blue-sensitive silver halide emulsion layer of a multilayer color photographic light-sensitive material which has a red-sensitive silver halide emulsion layer, a green-sensitive silver halide emulsion layer and the blue-sensitive silver halide emulsion layer in this order from a support, although the sharpness of lower layer(s) is/are certainly improved, the reflection of blue light at the blue-sensitive silver halide emulsion layer is strong and, thus, multiple reflection between the blue-sensitive layer and the surface of a film occurs, resulting in degradation of sharpness at the low frequency area in the blue-sensitive layer. An attempt at controlling the thickness of tabular silver halide grains in order to avoid such reflection of blue light leads to increase in the relfection of green light or red light, and results in deterioration of the sharpness in the low frequency area of magenta images or cyan images.
On the other hand, when tabular silver halide grains having a very thin thickness as used to minimize the reflection of visible light due to the tabular silver halide grains in the blue-sensitive emulsion layer, the absorption of blue light decreases as described above and blue sensitivity becomes insufficient. In order to compensate for such a decrease in blue sensitivity, it is necessary to employ blue-sensitive tabular silver halide grains having a large size. However, this can bring about an adverse affect such as an increase in graininess.
As described above, although sharpness in the high frequency area is improved by employing tabular silver halide grains in a light-sensitive silver halide emulsion layer, sharpness in the low frequency area conversely tends to deteriorate.
Moreover, in the case of using a tabular grain silver halide emulsion, there is the tendency that an edge effect is obtained to a lesser extent as compared with the use of a spherical grain silver halide emulsion. It is believed that this is because development inhibition from highly exposed areas to low exposure area is lessened due to the high developing speed of tabular grain silver halide emulsions at the initial stage of development. Since the edge effect is hardly obtained with tabular silver halide grains, sharpness in the low frequency area is inferior in comparison with spherical silver halide grains.
According to the investigations by the inventors, it has been found that image sharpness in the low frequency area can be improved by increasing in the interimage effect between light-sensitive silver halide emulsion layers. Some technique for increasing the interimage effect are known. For example, it is described in U.S. Pat. No. 3,536,486 that a preferred interimage effect is obtained by introducing a diffusible 4-thiazolin-2-thione into an exposed color reversal photographic element. Also, a method for obtaining a preferred interimage effect by introducing a diffusible 4-thiazolin-2-thione into an unexposed color reversal photographic element is described in U.S. Pat. No. 3,536,487. Further, in Japanese Patent Publication No. 34169/73, it is described that a remarkably high interimage effect is achieved by the presence of an N-substituted 4-thiazolin-2-thione compound when silver halide is reduced to silver upon development of a color photographic light-sensitive material.
However, with these methods described above, the interimage effect obtained and the improvement in sharpness in the low frequency area are extremely small and are insufficient.
Image sharpness is visually very important not only in the high frequency area but also in the low frequency area. Thus, it has been desired to improve sharpness in both the high frequency are and in the low frequency area.